Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/sigs.k8s.io/yaml/SECURITY_CONTACTS

@@ -0,0 +1,17 @@
+# Defined below are the security contacts for this repo.
+#
+# They are the contact point for the Product Security Team to reach out
+# to for triaging and handling of incoming issues.
+#
+# The below names agree to abide by the
+# [Embargo Policy](https://github.com/kubernetes/sig-release/blob/master/security-release-process-documentation/security-release-process.md#embargo-policy)
+# and will be removed and replaced if they violate that agreement.
+#
+# DO NOT REPORT SECURITY VULNERABILITIES DIRECTLY TO THESE NAMES, FOLLOW THE
+# INSTRUCTIONS AT https://kubernetes.io/security/
+
+cjcullen
+jessfraz
+liggitt
+philips
+tallclair